This invention is directed to a thermal coupling structure particularly for the thermal coupling between a refrigerated sensor and a refrigeration source.
Where a device is to be cooled, it is necessary to provide a thermal pathway between the device and the source of refrigeration. When cryogenic temperatures are to be reached (at or below liquid nitrogen temperatures) thermal coupling becomes more difficult. Several factors present difficult coupling. One of the factors is the need for careful insulation of the cold zone. Such insulation is often in the form of a dewar which is a double walled structure having an evacuated space. The evacuated space reduces thermal loss by conduction. Quite often the dewar is made of glass and in such case, the glass is silvered to minimize thermal conduction by radiation. A cooler is provided within the interior tube of the dewar, and with thermal changes such a glass dewar can receive stresses which are destructive. A metal bellows filled with a thermally conductive material is employed as the thermal coupling device in G. P. Lagodmos, U.S. Pat. No. 3,807,188. C. M. Bower et al, U.S. Pat. No. 3,999,403 teaches a metal bellows which is intended to maintain elasticity at cryogenic temperatures. Both of those patents show a cold finger in which cooling is produced, together with a device to be cooled and the therma coupling bellows. R. C. Longsworth, U.S. Pat. No. 3,728,868 also uses a bellows, see FIG. 3.
Another common thermal connection between a cold finger and a detector is a pad of copper wool loaded with thermal grease positioned between these parts. At cryogenic temperatures the grease freezes making the conductive paths solid. Metal wool is employed to enhance thermal conduction in both P. J. Walsh, U.S. Pat. No. 3,315,478 and in K. E. Nicholds, U.S. Pat. No. 3,704,579. However, in both of these cases the cryogenic liquid product of expansion from a Joule-Thomson valve is discharged directly into the wool where it boils to provide the refrigeration.
K. W. Cowans, U.S. Pat. No. 3,306,075 teaches a plurality of spring fingers around a substantial length of the cold finger in contact with the inner dewar wall to provide thermal contact. In that structure a plurality of metallic cantilevered leaves attached to the finger are resiliently expanded by a separate spring. These leaves extend over substantial length of the cold finger. The problem with this is that most cold fingers are at their lowest temperature closest to their tip, and the temperature is graded along the length thereof.
Each of these prior structures presents one or more problems in thermal coupling so that an improved coupling structure is required.